Semiconductor transistors, in particular field-effect controlled switching devices such as a Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) or an Insulated Gate Bipolar Transistor (IGBT) have been used for various applications including but not limited to use as switches in power supplies and power converters, electric cars, air-conditioners, and even stereo systems. Particularly with regard to power devices capable of switching large currents and/or operating at higher voltages, low on-state resistance, which is in the following also referred to as on-resistance Ron, and high voltage blocking capability are often desired. Due to structural efficiency and low on-resistance Ron, vertical trench MOSFETs are widely used, in particular in power applications. The breakdown voltage of a trench MOSFET may be raised by optimizing the shape and depth of the vertical trench and in particular by arranging an insulated field electrode in the vertical trench below the insulated gate electrode. The field electrode is typically connected to source potential and insulated form drain potential by a field oxide. Accordingly, charges in the drift region between two vertical trenches are at least partially compensated during blocking mode and thus the blocking capability improved.
To avoid reducing of break-down field strength in a peripheral area surrounding an active area with active MOSFET-cells and IGBT-cells, respectively, edge-termination structures may be provided in the peripheral area reducing the electric field in lateral direction during blocking mode. However, high current densities may occur at the outermost active cell during hard commutating and switching conditions, respectively, of for example more than about 1010 V/s. This may result in failure of the semiconductor device.
Accordingly, there is a need to provide semiconductor transistors which have a higher dynamic robustness, i.e. which are more robust during commutating.